Regional and Localized Fluid Flow in the Southern Irish Midlands: A Hybrid Fluid-Flow Model for the Rathdowney Trend

Regional sedimentological, petrologic, geochemical, and fluid inclusion studies of Lower Carboniferous carbonate rocks of the Irish Midlands and Dublin Basin reveal a complex history of dolomitization and mineralization resulting from interactions among at least three chemically, and thermally distinct fluids. Total strontium, delta O-18 and Sr-87/Sr-86 values of saddle dolomite cements and Cl/Br and T-h values of their included fluids exhibit systematic variation. End-member compositions consist of: 1) High total strontium, low Cl/Br ratio, low Sr-87/Sr-86, low- to moderate-salinity fluids; and 2) Variable total strontium, higher Cl/Br ratio, low Sr-87/Sr-86, low- to moderate-salinity fluids. Cements dominated by low temperature, high-salinity fluids display intermediate values. The delta O-18-Cl/Br systematics of dolomite cements and included fluids indicate extensive seawater evaporation. We have identified Lower Carboniferous carbonate units that contain evidence of evaporite sequences, suggesting that dense brines may have formed adjacent to the Leinster Massif. These areas may represent source areas for highly saline brines associated with dolomitization and mineralization in the Rathdowney Trend. Evaporite-associated dolomites are gradually enriched in O-18 downward through the underlying strata, consistent with vertical brine migration. These data are consistent with a fluid flow model that incorporates both regionally extensive evaporative brines and local, basement-involved hydrothermal convection.